1. Field of the Invention
The present invention relates to a solid electrolytic capacitor comprising a porous sintered body made of a valve metal, and also relates to a manufacturing method of making the same.
2. Description of the Related Art
An example of a solid electrolytic capacitor includes a capacitor utilized for canceling noise generated by a device such as CPU, or for stabilizing power supply for an electronic device. (Refer to JP-A-2003-163137, for example.) FIG. 19 illustrates an example of such a solid electrolytic capacitor. The illustrated solid electrolytic capacitor X includes a porous sintered body 90 made of a metal having valve action. The porous sintered body 90 is provided with an anode wire 91 partly protruding out thereof. The surface of the porous sintered body 90 is formed with a cathode conductive layer 92. Each of the anode wire 91 and the conductive layer 92 is connected to a respective one of conductive members 93, 94 partly protruding from a sealing resin 95 to serve as an external anode terminal 93a and an external cathode terminal 94a for surface mounting. The frequency characteristic of the impedance Z at the solid electrolytic capacitor is given by the following formula.Z=√{square root over ((R2+(1/ωC−ωL)2))}                (ω: 2πf (f: frequency), C: capacity, R: resistance, L: inductance)        
As seen from the above formula, in a low frequency range in which the frequency is lower than the self-resonance point, “1/ωC” is dominant, where the impedance can be lowered by increasing the capacity of the solid electrolytic capacitor X. In a high frequency range in which the frequency is around the self-resonance point, the resistance “R” is dominant, where it is required to lower ESR (equivalent series resistance) of the solid electrolytic capacitor X. Further, in an ultrahigh frequency range in which the frequency is higher than the self-resonance point, “ωL” is dominant, where it is required to lower ESL (equivalent series inductance) of the solid electrolytic capacitor X.
Recently, noise at a high frequency including high harmonic component is generated by a device having a high clock frequency, such as a CPU. Further, in accordance with an increase in operation speed and digitalization of electronic devices, a power supply system with high responsiveness has become necessary. For such use, the solid electrolytic capacitor X is also strongly desired to have a lowered ESL. In order to lower the ESL, the shape of the porous sintered body 90 may be changed, or a plurality of anode wires 91 may be provided. However, the conductive path between the external anode terminal 93a and the porous sintered body 90 includes a vertical portion 93b, at the conductive member 93, that extends perpendicular to the longitudinal direction of a circuit board on which the solid electrolytic capacitor X is to be mounted. Through the vertical portion, the electrical current passes in a direction different from the adjacent portions of the conductive path. Thus, the length of the vertical portion is proportional to the inductance, and thus to the impedance at a high frequency range. Therefore, the solid electrolytic capacitor X cannot have a sufficiently lowered ESL.